An ink ribbon cassette containing therein an ink ribbon has been conventionally widely used so that the ink ribbon can be easily mounted on a printer and an operator's hand and the printer are prevented from being contaminated when the ink ribbon is mounted on the printer.
The life of the ink ribbon is in general determined by the amount of ink which is impregnated into the ink ribbon. However, the life of a ground fabric of the ink ribbon is normally longer than that of the ink ribbon. Accordingly, there is proposed an ink ribbon cassette provided with an ink supplying means for supplying ink to the ink ribbon in order to lengthen the life of the ink ribbon.
FIG. 1 is an exploded perspective view of the ink ribbon cassette wherein an upper cover of the ink ribbon cassette and an upper cover of an ink occluding body case are respectively removed, based on which the conventional ink ribbon cassette will be described hereinafter.
In the same figure, an ink ribbon 1, which is endless and looped, is contained in a ribbon containing portion 3 of an ink ribbon cassette 2 and the part of the ink ribbon 1 is exposed outside of the ink ribbon cassette 2 from the forward ends of ribbon guide portions 4a and 4b. The ink ribbon 1 is moved in the direction of the arrow at the exposed portion.
A driving gear 5 is disposed in an inlet of the ribbon containing portion 3 and is rotatably supported by a driving gear supporting member 6.
A follower gear 7 is rotatably supported by a follower gear supporting member 8 and is pressed against the driving gear 5 by a spring 9. Teeth of the follower gear 7 mesh with teeth of the driving gear 5 while clamping the ink ribbon 1 therebetween. An ink supplier 10 formed of felt is brought into contact with the follower gear 7 at one end thereof and contacts an ink occluding body 11 formed of fiber impregnated with ink at the other end thereof so as to draw ink from the ink occluding body 11 owing to the capillary action and to supply ink to the follower gear 7.
An ink occluding body cover 13 is to be attached to an ink occluding body case 12 to prevent ink in the ink occluding body 11 from leaking out.
A ribbon cassette cover 14 accommodates all these components in the ribbon cassette 2 and covers the ink ribbon cassette 2.
With such an arrangement as set forth above, the ink ribbon 1 which is clamped by the driving gear 5 and the follower gear 7 is driven by way of the driving gear 5 when a carriage performs space driving and is circulated and drawn in the ribbon containing portion 3 whereby the ink ribbon I is contained in the ribbon containing portion 3 as it is folded.
The ink ribbon I drawn out from the ribbon containing portion 3 is passed through the ribbon guide portion 4a and is once exposed outside of the ribbon cassette 2 and thereafter printing is performed through the ink ribbon 1. Upon completion of printing, the ink ribbon 1 is passed through the ribbon guide portion 4b and circulated and drawn into the ribbon containing portion 3.
When the ink ribbon I is drawn into the ribbon containing portion 3, ink from the ink supplier 10, which is brought into contact with the follower gear 7, is supplied to the follower gear 7 through which ink is transferred to and replenished to the ink ribbon 1.
However, as illustrated in FIG. 2 showing the relation between the number of printed letters and print density, i.e. print contrast signal (hereinafter referred to as PCS), PCS is lowered at the time A at a relatively early stage of printing. This leads to such a problem that PCS is not stabilized at the time A of the relatively early stage of printing.
The reason why PCS is lowered at the time A is that the amount of ink to be supplied from the ink occluding body 11 to the ink supplier 10 does not overtake the amount of consumption of ink which is impregnated beforehand into the ink ribbon. This is described more in detail with reference to FIG. 3.
FIG. 3 is a graph showing the result of investigation of PCS using individually the ink ribbon and the ink occluding body. Printing by the ink ribbon 1 alone means that the ink supplier 10 is removed and ink in the ink occluding body 11 is not supplied to the ink ribbon 1 while printing using the ink occluding body 11 alone means that a blank ribbon, which is not impregnated with ink, is used. Print density using the ink ribbon 1 alone and the ink occluding body 11 alone are totalled to indicate PCS in FIG. 2. As illustrated in FIG. 3, printing using the ink ribbon 1 alone shows a sharp dropping of PCS while printing using the ink occluding body 11 alone shows that PCS increases between the beginning of printing and five hundred thousand printed letters. Since the intersecting point between the line showing PCS of the ink ribbon 1 alone and the line showing PCS of the ink occluding body 11 alone shows that the number of printed letters are two hundred thousands, the reason why PCS in normal printing in FIG. 2 is lowered between one hundred thousand printed letters and two hundred thousand printed letters and is increased from five hundred thousand printed letters is that PCS using the ink ribbon 1 alone is sharply dropped and PCS using the ink occluding body 11 alone is greater than PCS using the ink ribbon 1 alone after two hundred thousand printed letters and remains increased until five hundred thousand printed letters.
There is known an improvement of another ink replenishing type ink ribbon cassette which is disclosed in a gazette of Japanese Utility Model Laid-Open Publication No. 63-84362. The ink ribbon cassette as disclosed in the gazette has an auxiliary ink supplying member in addition to a main ink supplying member for supplying ink to the ink ribbon wherein the auxiliary ink supplying member can be switched so as to be connected to or disconnected from the main ink supplying member by a switching member. That is, according to this ink ribbon cassette, the thickness of the ink supply route can be switched depending on the amount of ink residual in an ink impregnated body.
However, there are problems in this ink ribbon cassette. That is, firstly, when the auxiliary ink supplying member is forced to contact the main ink supplying member by the switching member after PCS is lowered (at point B) as illustrated in FIG. 4, the amount of ink is sharply reduced thereafter so that PCS is sharply dropped. Secondly, when the auxiliary ink supplying member is forced to contact the main ink supplying member from the beginning of printing, PCS is too high at the early stage of printing as illustrated in FIG. 5 and thereafter PCS is lowered at the relatively early stage of printing, which results in shortening the life of the ink ribbon. Thirdly, since the switching member should be manually operated, the switching operation is troublesome.
Accordingly, it is an object of the present invention to provide an ink ribbon cassette which solves the unstableness of PCS at the early stage of printing without resorting to the manual operation for switching the ink supplying means.